- TUTORIALS: SHARPNESS -

Sharpness describes the clarity of detail in a photo, and can be a valuable
creative tool for emphasizing texture. Proper photographic and post-processing
technique can go a long way towards improving sharpness, although sharpness
is ultimately limited by your camera equipment, image magnification and viewing
distance. Two fundamental factors contribute to the perceived sharpness
of an image: resolution and acutance.

Acutance

Resolution

High

Low

High

Low

Acutance describes how quickly image information
transitions at an edge, and so high acutance results in sharp transitions
and detail with clearly defined borders.

Resolution describes the camera's ability
to distinguish between closely spaced elements of detail, such as the
two sets of lines shown above.

For digital cameras, resolution is limited by your
digital sensor,
whereas acutance depends on both the quality of your lens and the type of post-processing.
Acutance is the only aspect of sharpness which is still under your control after
the shot has been taken, so acutance is what is enhanced when you digitally
sharpen an image (see
Sharpening using an "Unsharp Mask").

COMPARISON

Photos require both high acutance and resolution to be perceived as critically
sharp. The following example is designed to give you a feel for how each
influences your image:

Acutance: High

Resolution: Low

Acutance: Low

Resolution: High

Acutance: High

Resolution: High

PROPERTIES OF SHARPNESS

Sharpness also depends on other factors which influence our perception of
resolution and acutance.
Image noise (or film grain) is usually detrimental to an image, however
small amounts can actually increase the appearance of sharpness. Consider
the following example:

Low Noise, Soft

High Noise, Sharp

Although both images have not been sharpened, the image to the left appears
softer and less detailed. Image noise can be both very fine and have a
very high acutance-- tricking the eye into thinking sharp detail is present.

Sharpness also depends on viewing distance. Images which are designed
to be viewed from further away, such as posters or billboards, may have much
lower resolution than fine art prints in a gallery, but yet both may be perceived
as sharp because of your viewing distance. Keep this property in mind
when sharpening your image, as the optimal type of your sharpening may not necessarily
be what looks best on your screen.

Sharpness is also significantly affected by your camera technique.
Even small amounts of camera shake can dramatically reduce the sharpness of
an image. Proper shutter speeds, use of a sturdy
camera tripod and mirror lock-up can also significantly impact the sharpness
of your prints.